Communication satellites are placed into geosynchronous orbits in which the satellite rotates about the earth at the same rate, and in the same direction, that the earth turns, thus maintaining the satellite above a particular point on the earth. These orbits are in the earth's equatorial plane at approximately 22,300 miles above the earth's surface. Antenna systems for receiving the signals transmitted by these satellites fall into two basic categories: fixed systems and mobile systems. A typical fixed antenna system is that used for receiving satellite television transmissions in one's home. A typical mobile system is a "satellite truck" used by television stations to relay information from a site remote from the television studio.
If a fixed antenna is to receive signals from only one satellite, then of course it is only necessary to point the antenna at the satellite and secure the antenna in place. If a fixed antenna is to receive signals from more than one satellite, the antenna is generally rotatable about the polar axis of the system, with the antenna tilted and fixed with respect to the polar axis at the proper declination angle. Mobile antennas, which must be repositioned both in azimuth and in elevation for different locations on the earth's surface, generally employ computer controlled servomechanisms which drive the antenna in response to inputs of antenna location and satellite location.